How did Lamarck’s ideas challenge the belief that species were fixed and unchanging?

Jean-Baptiste Lamarck challenged the prevailing belief that species were fixed and unchanging by proposing a comprehensive theory of organic transformation, which he first presented in 1800.1

Lamarck argued that species were not static but instead underwent change over time as they adapted to their environments. His theory relied on two primary mechanisms:12
* The power of life: An inherent tendency for organisms to increase in complexity.1
* Environmental influence: The idea that organisms respond to changing circumstances by adopting new habits.1

Lamarck posited that these new habits caused certain organs to be used more or less, leading to physical changes—a concept known as the inheritance of acquired characters. He suggested that these acquired traits were passed on to offspring, and that small changes accumulating over vast periods resulted in the diversity of life observed today. By systematically arguing that species could transform through these natural processes, Lamarck provided an early, detailed framework for evolution that directly contradicted the notion of immutable, fixed species.132